The present invention relates to an image forming apparatus that forms a toner image by use of a liquid developer in which toner is dispersed in a solvent.
An image forming apparatus is exemplified by a copying machine which utilizes the electrostatic copying process. This type of copying machine forms an electrostatic latent image on a photosensitive body. The electrostatic latent image corresponds to image information on an original, i.e., an object to be copied, and the image information is sent to the photosensitive body as light-and-shade information. The electrostatic latent image is then visualized by the toner, i.e., a developer. As one method for supplying toner to the electrostatic latent image, the use of the liquid developer in which toner is dispersed in a solvent is known.
In comparison with an image forming apparatus using a dry developer, an image forming apparatus using a liquid developer is advantageous in that the toner it uses is made of small-diameter particles. The use of such toner helps improve the image quality (high image quality), and increases the image forming speed. In addition, the liquid developer is advantageous in that it ensures improved gradation characteristics and enables production of images image that are very high in resolution like printed process. Moreover, the toner has a low melting point, and a toner image can be fixed easily.
However, since the developer is in the liquid state and includes toner particles and a carrier liquid, there is still room for improvement with respect to the method in which the developer is supplied and the method in which a residual developer liquid (a carrier liquid functioning as a solvent) remaining on a photosensitive body is removed. In particular, this holds true for an image formation method in which a color image is obtained by superposing toner images of three or four colors on a photosensitive body and then simultaneously transferring them onto a transfer material. In this image formation method, the developer image of the first color must not mix with the developer images of the next colors after development of the first color. To achieve this, the amount of solvent included in the developer image on the photosensitive body has to be reduced, for example, by non-contact wring, contact wring, drying by air supply, etc.
A wet-type image forming apparatus using a liquid developer has problems in that the image density (image quality) of a toner image formed on the photosensitive body and that of an image (toner) transferred onto a transfer material are not stable.
To solve this problem, a patch of predetermined size (a test image) is formed on a photosensitive body. After the patch is subject to development, the amount of toner attached to the patch is measured, and the measurement is fed back when a toner replenishment operation is performed or when a toner consumption condition (developing condition) is determined. This kind of control is well known in the art.
For example, U.S. Pat. No. 4,082,445 discloses a method in which the amount of toner attached to a photosensitive body is measured by checking the amount of light reflected from a non-image portion of the image bearing member (i.e., the photosensitive body) and the amount of light reflected from a toner layer obtained by developing a latent image and comparing the difference between these amounts with a reference value.
It should be noted, however, that the amount of toner attached to the photosensitive body (i.e., the total amount of toner constituting a toner layer) and the absorption index of light do not vary linearly (non-linear). Let us assume that the surface of the photosensitive body is completely covered with toner particles. In this state of (toner) layer, the reflection factor of light hardly varies without reference to the number of toner layers formed.
In contrast, in the state where the surface of the photosensitive body is partly exposed between toner layer portions, the reflection by the surface of the photosensitive body is inevitably sensed as the reflection factor of the light falling on the toner layer. This being so, the amount of toner attached cannot be accurately measured. It should be also noted that the reflection by the surface of the photosensitive body is dependent on changes in the surface roughness of the photosensitive body, changes in the thickness of the photosensitive layer of the photosensitive body, the occurrence of filming of toner, etc.
With respect to color toner, there may be a case where the thickness of a toner layer cannot be measured, depending upon the combination between the spectral reflection characteristics of the toner and the surface of the photosensitive body. In addition, the wavelength of measurement light must be changed in accordance with the color of toner.
As can be seen from the above, the method shown in U.S. Pat. No. 4,082,445 does not necessarily enable accurate measurement of the amount of toner attached to the photosensitive body.
Jpn. Pat. Appln. KOKAI Publication No. 8-327331 discloses a developer amount measuring method for use in a dry-type image forming apparatus. This method employs a lens that provides different image formation positions in accordance with the thickness of a toner layer. An optical position-detecting element detects the variation (difference) in the image formation positions of the lens.
However, toner particles used in wet-type development are very fine; they are in the range of 0.2 to 1.5 xcexcm. Even if the amount of toner attaching on a transfer medium changes to such an extent as to change the reflecting density of an image transferred onto that medium, the position where light is focused on the toner layer does not significantly change. Therefore, a change in the amount of toner adhering is hard to detect.
Jpn. Pat. Appln. KOKAI Publication No. 8-87144 shows a method for measuring the thickness of a toner layer. According to the publication, the potential of the toner layer obtained by development is measured to detect the thickness of the toner layer. The publication does not describe anything regarding wet-type development, and in view of the drawings and an embodiment, the publication is considered to relate to dry-type development.
However, the toner used for wet-type development does not contain a high proportion of resin. In comparison with the toner used for dry-type development, the potential of a toner layer formed with the toner for wet-type development is likely to be affected by pigments. The toner used for wet-type development does not produce such a toner layer thickness as shown in Jpn. Pat. Appln. KOKAI Publication No. 8-87144. In particular, when magenta toner is used, there is no substantial potential difference between a toner layer and a latent image. Even if the potential at the toner layer is utilized, the amount of toner attaching cannot be detected with high precision.
As discussed above, none of the techniques shown in the known publications enable accurate detection of the thickness of a toner layer formed on a photosensitive body after development as long as the development is wet-type development using fine toner particles whose average particles diameter is in the range of 0.2 to 1.5 xcexcm. (In other words, there is no established detection method.) In particular, in the case of wet-type development of color images, wherein toner images of three or four colors are superposed on a photosensitive body and are simultaneously transferred to a transfer medium, the development of each color is followed by the reduction of the amount of solvent contained in the developer image formed on the photosensitive body, so as to prevent color mixing. For example, the non-contact or contact wring of the solvent, the drying of the solvent by air supply, etc. are repeatedly executed, so that the developer image of each color may not mix with the developer images of the subsequent colors after development of each color. For this reason, the sensing of the thickness of a toner layer is much more difficult.
An object of the present invention is to provide an image forming method and an image forming apparatus that are capable of measuring the amount of developer contained in a wet-type electrophotographic developing agent on a photosensitive body or a transfer medium. The present invention has been made in consideration of the above problems and provides an electrophotographic developing apparatus which develops a latent image on a photosensitive body by use of a toner liquid in which toner is dispersed in a nonpolarity solvent and which thereby forms a toner image on the photosensitive body, the electrophotographic developing apparatus comprising:
a photosensitive body which can hold an electrostatic image;
a charging unit which provides the photosensitive body with a predetermined potential;
an exposure unit which forms an electrostatic image on the photosensitive body;
a developing unit which includes a developing roller opposing the photosensitive body with a predetermined gap maintained, and which supplies a toner liquid to the electrostatic image formed on the photosensitive body such that toner is selectively attached to the electrostatic image, thereby forming a toner image;
a developing bias source which can apply a predetermined bias voltage to the developing unit;
a toner layer thickness-detecting mechanism which detects the thickness of toner constituting the toner image, the toner layer thickness-detecting mechanism including an emission system which emits polarized light toward a toner image formed by the developing unit, and a reception system which receives the polarized light reflected by the toner image and produces an electric signal; and
an image formation condition-controlling device which controls at least one of an output from the charting unit, an output from the exposure unit and the bias voltage applied by the developing bias source, on the basis of the thickness of the toner layer detected by the toner layer thickness-detecting mechanism.
The present also provides an electrophotographic developing method which develops a latent image on a photosensitive body by use of a toner liquid in which toner is dispersed in a non-polarity solvent and which thereby forms a toner image on the photosensitive body, the electrophotographic developing method comprising:
providing the photosensitive body with a predetermined potential;
forming a test image by exposure by use of an exposure unit;
forming a toner layer on the photosensitive body by developing the test image;
irradiating the toner layer with polarized light and measuring the thickness of the toner layer on the basis of reflected light reflected by the toner layer by use of a layer thickness-detecting mechanism, the toner layer thickness-detecting mechanism including an emission system which emits polarized light and a reception system which receives the reflected light and produces an electric signal; and
comparing the thickness of the toner layer with a reference value and varying at least one of the potential provided for the photosensitive body, an intensity of exposure light output from the exposure unit, a toner density of the toner liquid and a developing bias voltage applied to a developing unit.
The present invention further provides a method which is based on subtractive primaries and forms a color image by using first-color toner, second-color toner and third-color toner, which produce first to third complementary colors to three primary colors and by further using seventh-color toner which emphasizes black, the method comprising:
rotating a photosensitive body at a predetermined rate;
charging the photosensitive body, which is capable of holding an electrostatic image thereon, to a predetermined potential that enables formation of a first-color toner image;
irradiating the photosensitive body with light corresponding to image data used for forming the first-color toner image;
forming the first-color toner image by supplying the first-color toner to an electrostatic image corresponding to the first-color toner image;
charging the photosensitive body, which is capable of holding an electrostatic image thereon, to a predetermined potential that enables formation of both a second-color toner image and a fourth-color toner image, the fourth-color toner image being an image obtained by superposing the first-color toner and the second-color toner;
irradiating the photosensitive body with light corresponding to image data used for forming the second-color toner image and the fourth-color toner image;
forming the second-color toner image and the fourth-color toner image by supplying the second-color toner to electrostatic images corresponding to the second-color toner image and fourth-color toner image;
charging the photosensitive body, which is capable of holding an electrostatic image thereon, to a predetermined potential that enables formation of a third-color toner image, a fifth-color toner image and a sixth-color toner image, the fifth toner image being an image obtained by superposing the second-color toner and the third-color toner, and the sixth-color toner image being an image obtained by superposing the first-color toner and the third-color toner;
irradiating the photosensitive body with light corresponding to image data used for forming the third-color toner image, the fifth-color toner image and the sixth-color toner image;
forming the second-color toner image, the fifth-color toner image and the sixth-color toner image by supplying the second-color toner to electrostatic images corresponding to the second-color toner image, the fifth-color toner image and sixth-color toner image;
charging the photosensitive body, which is capable of holding an electrostatic image thereon, to a predetermined potential that enables formation of a seventh-color toner image, the seventh-color toner image being formed without reference to an order in which the first to sixth-color toner images are formed;
irradiating the photosensitive body with light corresponding to image data used for forming the seventh-color toner image;
forming the seventh-color toner image by supplying the seventh-color toner to an electrostatic image corresponding to the seventh-color toner image;
detecting the thickness of each of the toner layers by use of a toner layer thickness-detecting device in a state where the photosensitive body is rotating, the toner layer thickness-detecting device including an emission system which emits polarized light toward each of the toner images, and a reception system which receives the polarized light reflected by each toner image and produces an electric signal;
comparing the thickness of each of the toner layers with a reference value; and
varying at least one of a potential provided for the photosensitive body to obtain a desired image, the amount of light output from an exposure unit to obtain the desired toner image, a toner density of a given toner liquid and a developing bias voltage applied to a developing unit containing the given toner liquid, in accordance with a result of comparison.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.